David Cameron announced a dramatic expansion of his 'troubled families' programme today as he claimed it had "turned around" the lives of nearly all those who took part.

In a wide-ranging speech on welfare reform, the Prime Minister revealed that nearly all of the 117,000 families targeted by the scheme now had either adults in work or their children in regular schooling, or both.

The project would be expanded to 400,000 more families, he said.

But critics soon pounced on Mr Cameron's claims that the programme had saved the taxpayer £1.2bn.

He also faced a backlash from campaigners worried by a fresh threat to tax credits and a strong hint that he would change the definition of child poverty.

Under the project run by 'troubled families czar' Louise Casey, households vulnerable to truancy, crime, anti-social behaviour, unemployment or domestic violence are helped by councils and Whitehall.

The co-ordinated programme targets those families most at risk of multiple problems, with 71% having a physical health problem and 46% a mental health issue.

The Prime Minister said: "Our troubled families programme, under Louise Casey, has changed lives. By radically changing the way we deliver services to the hardest-to-reach families in our country, we have tackled worklessness, addiction, truancy and anti-social behaviour.

"I can announce today that almost all of the 117,000 families which the programme started working with have now been turned around – in terms of either school attendance or getting a job or both.

"This has saved as much as £1.2 billion in the process. And in the next five years, we will work with 400,000 more."

Mr Cameron said that for years many thousands of hard-to-reach families had had visits from police or social workers but the Troubled Families programme actually sent specialists into the home to provide support.

"It's not that they don't get intervention from the state. They get buckets of intervention. But what they don't get is someone working in the family. And that's what the troubled families programme has delivered. In some ways, it's like old fashioned social work that's been rediscovered and that's no bad thing."

But within minutes of his speech, Jonathan Portes, Director, National Institute of Economic and Social Research (NIESR) told The Huffington Post UK that he stood by his claim that the purported savings were 'pure unadulterated fiction'.

In a blog for NIESR published earlier this year over similar claims, Mr Portes said 'I doubt the North Korean Statistical Office would have the cheek' to put out such statistics.

Asked if he would rule out further disability benefit cuts, Mr Cameron replied only that the Government was spending more on such state help than in the past.

The Prime Minister also gave a strong hint that he wanted to adopt Iain Duncan Smith's plans to redefine the measurement of child poverty, a move that was shelved before the election.

Declaring that it was time to treat the causes of social problems and not just their 'symptoms', he said the relative measure of poverty caused distortions.

"Just take the historic approach to tackling child poverty. Today, because of the way it is measured, we are in the absurd situation where if we increase the state pension, child poverty actually goes up."

Newly-elected Work and Pensions Select Committee chairman Frank Field, who has today written a HuffPostUK blog on his local schemes to combat child hunger, said that he welcomed the Prime Minister’s focus on the causes of child poverty.

"But this was a central question posed in the report he asked from me in 2010 on poverty and life chances. All that’s lacking is government action. I’d hope in this parliament we’ll move beyond talking and actually get some action from the Government to prevent poor children becoming poor adults."